Revert Chat/Agent mode tag in message header

Per user report — the tag's mode metadata coincided with a
500 error on agent mode (especially on mobile). Removing the
UI tag, the chat.js writes of metadata.mode, and the CSS pill
so agent mode posts work cleanly again.

Touches:
- chat.js: drop _sendMode capture + meta.mode writes (user + assistant)
- chatRenderer.js: roleTimestamp() back to a single (when) arg, drop
  the .role-mode-tag append; updated three call sites
- style.css: dropped .role-mode-tag and .role-mode-agent rules

static/style.css

@@ -3247,26 +3247,6 @@ body.bg-pattern-sparkles {
     .role-timestamp {
       font-size:0.7rem; color:var(--color-muted-alt); font-weight:normal; margin-left:6px;
     }
-    /* Mode tag (Chat / Agent) sits right after the timestamp so users
-       can tell at a glance which path a message took. Subtle pill —
-       no accent background, just text + thin border. */
-    .role-mode-tag {
-      display: inline-block;
-      margin-left: 6px;
-      padding: 0 5px;
-      font-size: 0.62rem;
-      line-height: 1.6;
-      font-weight: 600;
-      text-transform: uppercase;
-      letter-spacing: 0.4px;
-      border-radius: 999px;
-      border: 1px solid color-mix(in srgb, var(--fg) 18%, transparent);
-      opacity: 0.7;
-    }
-    .role-mode-tag.role-mode-agent {
-      color: var(--accent, var(--red));
-      border-color: color-mix(in srgb, var(--accent, var(--red)) 35%, transparent);
-    }
     .msg-footer {
       display:flex; align-items:center; gap:6px;
       flex-wrap: wrap;